I deleted the 'Default Tree' and afterwards the graphs tab was blank. No graphs were displayed.
After hours of reading many websites and play with sql-entries I found the solution.
In the database 'cacti' and the table 'graph_tree' at least one entry must have the id '1'. As default the 'Default Tree' has the id '1'.
I don't know if it's a bug or a feature...
kind regards
Martin